In general, organopolysiloxane-utilized release agents are classified by types of the curing reactions they induce. Hitherto, the so-called addition reaction type organopoly-siloxane compositions comprising alkenyl group-containing organopolysiloxanes, organohydrogenpolysiloxanes and platinum catalysts have prevailingly utilized in release agents for, e.g., adhesive labels and adhesive tapes.
For utilization of addition reaction in silicones for released paper use, the foregoing compositions require control agents that, though inhibit the reaction of platinum catalysts at room temperature, ensure rapid progress of the reaction between alkenyl group-containing organopolysiloxanes and organohydrogenpolysiloxanes under high temperatures. On such control agents, there have already been various proposals and it has been disclosed that acetylene alcohols are useful in particular (Japanese Tokko Sho 44-31476, Japanese Tokkai Hei 6-329917 and Japanese Tokkai Hei 9-143371, wherein the term "Tokko" as used herein means "an examined patent publication" and the term "Tokkai" as used herein means "an unexamined published patent application").
For the viewpoint of improving the productivity, on the other hand, high-speed curing under low temperatures has been required for silicone compositions in recent years. To meet such a requirement, the measure of increase in the amount of a catalyst used has often been taken. However, such a measure has a drawback of advancing the progress of gelation in a silicone treatment bath with the lapse of time although it can enhance initial curability. Even if the amount of a catalyst used is not increased in particular, gelation occurs similarly to the case of increasing the catalyst amount so long as the silicone treatment bath is left to stand under high temperatures for a long time.
With the intention of preventing such a gelation phenomenon, the use of an addition reaction controlling agent in an increased amount can lengthen the pot life, but lowers the initial curability. In the case where the addition reaction controlling agent is selected from the long-chain .alpha.-acetylene alcohols as disclosed in Japanese Tokkai Hei 6-329917, the pot life can be lengthened even if the amount used is small, but the initial curability is lowered.
On the other hand, there are known the control agents prepared by silylating the OH groups of acetylene alcohols (Japanese Tokkai Sho 61-261). These control agents cause no curability drop in silicone compositions even when their addition amount is increased so as to secure adequate control effect. However, the use of such an agent in a large amount causes a curability drop in the treatment bath with the lapse of time although it has no influence on the initial curability.
Further, it is disclosed in Japanese Tokkai Hei 9-143371 that the combined use of two control agents differing in boiling point can lengthen the time required for gelation in a thin-film state. Even when such a method is adopted, it is difficult to ensure a sufficient pot life and no curability drop in silicone compositions under a strict condition such that the amount of a catalyst used is increased or the treatment bath is left to stand at high temperatures.
Thus, there has been a strong request for control agents capable of ensuring both sufficient pot life and excellent curing properties in a silicone composition even when the catalyst amount is increased or the treatment bath is left under high temperatures.
Under these circumstances, we have made further studies on the control action of conventional acetylene alcohols and silylated compounds thereof. As a result thereof, the behaviors of such compounds in a silicone treatment bath have became clear.
More specifically, in the case of using an acetylene alcohol as control agent, the acetylene alcohol coordinates strongly with a platinum metal catalyst immediately after preparing the treatment bath to lower the catalyst activity. The acetylene alcohol-coordinated catalyst, though its activity as an addition reaction catalyst is lowered, cannot completely stop the addition reaction but still has an ability to advance catalytic cycles of the addition reaction.
However, since the triple bond of an acetylene alcohol is more liable to the addition reaction than the alkenyl group(s) in the siloxane, the effective quantity of the acetylene alcohol in the treatment bath is decreased with the lapse of time, and thereby the shortage of control effect is caused to result in gelation of the treatment bath. Although the time required for gelation can be lengthened by increasing the amount of an acetylene alcohol used, this measure renders the control effect too strong; as a result, the initial curability is lowered.
In the case of using a silylated acetylene alcohol as control agent, the silylated acetylene alcohol coordinates with a platinum metal catalyst to lower the activity of the catalyst similarly to acetylene alcohols, but it has weaker coordination strength than acetylene alcohols, so that the coordinated catalyst is inferior in ability to control the addition reaction. Thus, even when the silylated acetylene alcohols are used in a large amount, the change in initial curability is not so large. However, the use of silylated acetylene alcohol in a large quantity causes a drop in curability with the lapse of time because the triple bond of a silylated acetylene alcohol can undergo addition reaction and the consumption of SiH bonds proceeds via the addition reaction with a large number of triple bonds present in the treatment bath.
As a result of our intensive studies made on the assumption that the combined use of these two types of control agents can effect both long pot life and excellent curing properties, it has been found that the combined use of a small amount of acetylene alcohol and a large amount of silylated acetylene alcohol in a silicone treatment bath enables the acquisition of satisfactory initial curability, can prevent the lowering of curability even under a condition that the treatment bath has an increased catalyst content and is left to stand, and can minimize the change in release characteristics of a curable silicone composition; thereby achieving the present invention.